A separation method using a filtering membrane has a lot of advantages over a separation method based on heating or phase-changing. Among the advantages is the high reliability of water treatment since the water of desired purity can be easily and stably satisfied by adjusting the size of the pores of the filtering membrane. Furthermore, since the separation method using a filtering membrane does not require a heating process, the membrane can be used with microorganisms which are useful for separation process but may be adversely affected by heat.
The separation method may comprise a flat-type membrane and a hollow fiber membrane. Since a hollow fiber membrane module performs a separation process by means of a bundle of hollow fiber membranes, it is more advantageous than a flat-type membrane in terms of an effective area used for the separation process.
Typically, the hollow fiber membrane has been widely used in the field of microfiltration and ultrafiltration for obtaining axenic water, drinking water, super pure water, and so on. Recently, however, application of the hollow fiber membrane is being expanded to include wastewater treatment, solid-liquid separation in a septic tank, removal of suspended solid (SS) from industrial wastewater, filtration of river, filtration of industrial water, and filtration of swimming pool water.
The hollow fiber membrane may be classified into a composite membrane which is manufactured by coating a tubular braid woven by polyester or polyamide fiber with a polymer resin film; and a single-layer membrane which is manufactured only with polymer resin without a reinforcing member such as a tubular braid.
Since the composite membrane comprises the tubular braid as a reinforcing member, it has relatively good mechanical property (strength and elongation). However, since the material of the tubular braid is different from that of the film coated thereon, their adhesive strength is weak. Thus, if a physical impact, e.g., impact by bubbles for aeration cleaning, is continuously applied to the composite membrane, the tubular braid and the film coated thereon might be separated from each other and the quality of the filtrate produced through the water treatment might be lowered. Further, due to the thickness of the tubular braid, it is impossible to reduce the total thickness of the composite membrane below a certain value, which makes the composite membrane more disadvantageous in terms of the effective filtration area. For these reasons, a single-layer membrane is more actively studied than a composite membrane nowadays.
Generally, a single-layer membrane can be manufactured by means of NIPS (Non-solvent Induced Phase Separation) or TIPS (Thermally Induced Phase Separation).
According to the NIPS, a single-layer membrane is manufactured by using a method comprising the steps of: preparing a spinning solution by dissolving polymer resin in a good solvent; extruding the prepared spinning solution through a spinneret; and inducing coagulation of the spinning solution by bringing the extruded spinning solution into contact with a solution including a non-solvent.
The porous membrane manufactured in accordance with the NIPS has insufficient mechanical strength because it has asymmetric sponge structure including macro voids. Due to the low mechanical strength, the porous membrane cannot satisfy the compaction index of 0.5 or less which is generally required in this technical field. That is, if a certain level of pressure is applied to the porous membrane, the membrane is seriously shrunk so that its pores are distorted and finally plugged up. As a result, the water permeability of the porous membrane becomes considerably lower. Additionally, the porous membrane prepared in accordance with the NIPS has a problem of low rejection rate with respect to impurities due to its large nominal pore size.
The term “macro void”, as used herein, refers to a pore whose circumcircle has a diameter of 50 μm or more, and the term “sponge structure”, as used herein, refers to a three-dimensional net structure of solids. The sponge structure has pores separated from each other by the solids constituting the net structure.
On the other hand, according to the TIPS, a single-layer membrane is manufactured by using a method comprising the steps of: preparing a spinning solution by forcibly dissolving polymer resin in a poor solvent at a temperature above the phase-separation temperature, extruding the prepared spinning solution through a spinneret; and coagulating the spinning solution by bringing the extruded spinning solution into contact with a cooling solution of a temperature below the phase-separation temperature.
As illustrated in FIG. 1, the porous membrane manufactured in accordance with the TIPS includes no macro voids and has symmetric bead structure which is symmetric in a membrane-thickness direction. Thus, the porous membrane manufactured in accordance with the TIPS is more advantageous than the porous membrane manufactured in accordance with NIPS in terms of mechanical strength and impurity rejection rate.
The term “bead structure”, as used herein, refers to a structure including a plurality of spherical crystallites, i.e., solids of spherical or sphere-like shape, which are directly connected with one another or are indirectly connected with one another through stem-shaped solids. The term “symmetric bead structure”, as used herein, refers to a bead structure having no substantial difference between the outer and inner portions of a membrane.
In spite of the aforementioned advantages of the bead structure, however, the porous membrane manufactured in accordance with the TIPS does not have competitive water permeability and elongation at break due to its dense structure, a typical characteristic of the bead structure. The low water permeability reduces the filtration capacity of the porous membrane, and the low elongation at break increases the risk that the membrane would be damaged during an aeration cleaning process for preventing the membrane from being fouled.